The powder metallurgy art generally uses different standard temperature regimes for the compaction of a metal powder to form a metal component. These include chill-pressing (pressing below ambient temperatures), cold-pressing (pressing at ambient temperatures), hot-pressing (pressing at temperatures above those at which the metal powder is capable of retaining work-hardening), and warm-pressing (pressing at temperatures between cold-pressing and hot-pressing).
Distinct advantages arise by pressing at temperatures above ambient temperature. The tensile strength and work hardening rate of most metals is reduced with increasing temperatures, and improved density and strength can be attained at lower compaction pressures. The extremely elevated temperatures of hot-pressing, however, introduce processing problems and accelerate wear of the dies. Therefore, current efforts are being directed towards the development of metal compositions suitable for warm-pressing processes.
The U.S. Pat. No. 4,955,798 (Musella) describes warm compaction in general. According to this patent, lubricants generally used for cold compaction, e.g. zinc stearate, can be used for warm compaction as well. In practice, however, it has proved impossible to use zinc stearate or ethylene bissteararmide (commercially available as ACRA WAX®.), which at present are the lubricants most frequently used for cold compaction, for warm compaction. The problems, which arise, are due to difficulties in filling the die in a satisfactory manner.
The U.S. Pat. Nos. 5,744,433 (Storstrom et al) and 5,154,881 (Rutz) disclose metal powder compositions including amide lubricants which are especially developed for warm compaction. The U.S. Pat. No. 5,744, 433 discloses a lubricant for metallurgical powder compositions contains an oligomer of amide type, which has a weight-average molecular weight Mw of 30,000 at the most. In the U.S. Pat. No. 5,154,881 the amide lubricant consists of the reaction product of a monocarboxylic acid, a dicarboxylic acid and a diamine. Especially preferred as a lubricant is ADVAWAX.®. 450, which is an ethylenebisstearamide product.
Although the lubricants disclosed in these two patents are especially developed for warm compaction and work well in many cases it has been found that different problems are encountered when these lubricants are used in metal compositions intended for large scale production of sintered components.